"R" in the numeric designation signifies that the 1981 standard was reaffirmed (recertified) as an American National Standard in 1987, 1995 & 2005. No changes can be made to a standard during reaffirmation.
This standard is applicable to operations with the following: N, Pu, Pu, Pu, Pu, Am, Am, Am, Cm, Cm, Cm, Cm, Cf, and Cf. Subcritical mass limits are presented for isolated fissionable units. The limits are not applicable to interacting units.